The present invention relates to continuously variable transmissions and more particularly to a method for rapidly changing the transmission ratio.
Continuously variable transmissions (CVT""s) have been known in the art for a number of years. A typical continuously variable transmission is disclosed in U.S. Pat. No. 4,241,618 to Smirl, reissued as No. 31,361. One type of a continuously variable transmission typically includes a primary pulley and a secondary pulley, each pulley having a fixed sheave and a movable sheave. A belt is drivingly connected between the primary and secondary pulleys. Engine torque is provided to the primary pulley which transfers the torque through the belt to the secondary pulley. In order to obtain a new transmission ratio, pressure to one of the movable pulley sheave""s is decreased while pressure to the other movable pulley sheave is increased. This imbalance of pressure forces the belt clamped between these sheaves into a higher or lower transmission ratio as required.
There are some limitations with current continuously variable transmission technology. A conventional continuously variable transmission system has a slow response time when the driver of the vehicle opens the throttle quickly, thus calling for a rapid change in the transmission ratio. Because conventional CVT control systems have dictated that the primary pulley pressure must be maintained high enough to prevent slippage of the belt, the rate of change of the pressure ratio between the primary pulley and the secondary pulley is limited. This limited change in the pressure ratio limits the rate at which the continuously variable transmission ratio changes, thereby creating a delay in the continuously variable transmission response time. Therefore, it is an objective of the present invention to provide an improved method for rapidly changing the transmission ratio in a continuously variable transmission.
The method according to the present invention includes providing a continuously variable transmission having a first pulley engagable with an engine by a clutch device and a second pulley drivingly connected to the first pulley by a belt mounted between the first pulley and the second pulley. The first and second pulleys each have a fixed sheave and a movable sheave, a position of the movable sheaves of the first and second pulleys defining a transmission ratio. When a rapid transmission ratio change is required, the engine is disengaged from the first pulley by disengaging the clutch device. This allows the engine to use almost all of its power to rapidly achieve a target speed corresponding with the vehicle speed and the target transmission ratio. The clamping pressure on the first pulley is reduced to a very low value to allow rapid movement to a new ratio. Enough pressure is maintained on the first pulley so as to prevent excessive belt slip. The position of the movable sheave of the second pulley is then rapidly moved to a new position for obtaining a target transmission ratio. The movable sheave of the first pulley then receives increased clamping pressure which allows it to carry full engine torque. Finally, the engine is reengaged to the first pulley by reengaging the clutch device.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.